Computation-Communication Trade-offs and Sensor Selection in Real-time Estimation for Processing Networks

Recent advances in electronics are enabling substantial processing to be performed at each node (robots, sensors) of a networked system. Local processing enables data compression and may mitigate measurement noise, but it is still slower compared to a central computer (it entails a larger computational delay). However, while nodes can process the data in parallel, the centralized computational is sequential in nature. On the other hand, if a node sends raw data to a central computer for processing, it incurs communication delay. This leads to a fundamental communication-computation trade-off, where each node has to decide on the optimal amount of preprocessing in order to maximize the network performance. We consider a network in charge of estimating the state of a dynamical system and provide three contributions. First, we provide a rigorous problem formulation for optimal real-time estimation in processing networks in the presence of delays. Second, we show that, in the case of a homogeneous network (where all sensors have the same computation) that monitors a continuous-time scalar linear system, the optimal amount of local preprocessing maximizing the network estimation performance can be computed analytically. Third, we consider the realistic case of a heterogeneous network monitoring a discrete-time multi-variate linear system and provide algorithms to decide on suitable preprocessing at each node, and to select a sensor subset when computational constraints make using all sensors suboptimal. Numerical simulations show that selecting the sensors is crucial. Moreover, we show that if the nodes apply the preprocessing policy suggested by our algorithms, they can largely improve the network estimation performance.Comment: 15 pages, 16 figures. Accepted journal versio

Asynchrony in image analysis: using the luminance-to-response-latency relationship to improve segmentation

We deal with the probiem of segmenting static images, a procedure known to be difficult in the case of very noisy patterns, The proposed approach rests on the transformation of a static image into a data flow in which the first image points to be processed are the brighter ones. This solution, inspired by human perception, in which strong luminances elicit reactions from the visual system before weaker ones, has led to the notion of asynchronous processing. The asynchronous processing of image points has required the design of a specific architecture that exploits time differences in the processing of information. The results otained when very noisy images are segmented demonstrate the strengths of this architecture; they also suggest extensions of the approach to other computer vision problem

Study of a colliding laser-produced plasma by analysis of time and space-resolved image spectra

The interaction of two counter-propagating laser-produced plasmas was studied using simultaneous imaging and spectroscopic techniques. Spectrally-filtered time-gated ICCD imaging was used to obtain information about the spatial dynamics and temporal evolution of the collision process. While, time-resolved imaging spectroscopy was used to determine the spatial and temporal distributions of electron temperature and density within the interaction region. We examine specifically the interaction of plasmas whose parameters match those typically used in pulsed laser deposition of thin films. These low temperature plasmas are highly collisional leading to the creation of a pronounced stagnation layer in the interaction region

Electron and ion stagnation at the collision front between two laser produced plasmas

We report results from a combined optical interferometric and spectrally resolved imaging study on colliding laser produced aluminium plasmas. A Nomarski interferometer was used to probe the spatio-temporal distribution of electron densities at the collision front. Analysis of the resulting interferograms reveals the formation and evolution of a localized electron density feature with a well-defined profile reminiscent of a stagnation layer. Electron stagnation begins at a time delay of 10 ns after the peak of the plasma generating laser pulse. The peak electron density was found to exceed 10^19 cm^−3 and the layer remained well defined up to a time delay of ca 100 ns. Temporally and spectrally resolved optical imaging was also undertaken, to compare the Al^+ ion distribution with that of the 2D electron density profile. This revealed nascent stagnation of singly charged ions at a delay time of 20 ns. We attribute these results to the effects of space charge separation in the seed plasma plumes

Emission characteristics and dynamics of the stagnation layer in colliding laser produced plasmas

The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminum plasmas have been investigated using time and space resolved optical emission spectroscopies and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localized in the vicinity of the target surface 1 mm from the target surface while singly and doubly charged ions lie predominantly at larger distances, 1.5 and 2 mm, respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Dt ~ 80 ns). On the other hand, the excited neutrals were observed to form a V-shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer

Demo: real-time indoors people tracking in scalable camera networks

In this demo we present a people tracker in indoor environments. The tracker executes in a network of smart cameras with overlapping views. Special attention is given to real-time processing by distribution of tasks between the cameras and the fusion server. Each camera performs tasks of processing the images and tracking of people in the image plane. Instead of camera images, only metadata (a bounding box per person) are sent from each camera to the fusion server. The metadata are used on the server side to estimate the position of each person in real-world coordinates. Although the tracker is designed to suit any indoor environment, in this demo the tracker's performance is presented in a meeting scenario, where occlusions of people by other people and/or furniture are significant and occur frequently. Multiple cameras insure views from multiple angles, which keeps tracking accurate even in cases of severe occlusions in some of the views

Real-time observation of a coherent lattice transformation into a high-symmetry phase

Excursions far from their equilibrium structures can bring crystalline solids through collective transformations including transitions into new phases that may be transient or long-lived. Direct spectroscopic observation of far-from-equilibrium rearrangements provides fundamental mechanistic insight into chemical and structural transformations, and a potential route to practical applications, including ultrafast optical control over material structure and properties. However, in many cases photoinduced transitions are irreversible or only slowly reversible, or the light fluence required exceeds material damage thresholds. This precludes conventional ultrafast spectroscopy in which optical excitation and probe pulses irradiate the sample many times, each measurement providing information about the sample response at just one probe delay time following excitation, with each measurement at a high repetition rate and with the sample fully recovering its initial state in between measurements. Using a single-shot, real-time measurement method, we were able to observe the photoinduced phase transition from the semimetallic, low-symmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities was predicted based on earlier measurements at moderate excitation densities below the damage threshold. Our observations indicate that coherent lattice vibrational motion launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for tens of picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration.Comment: 22 pages, 4 figure

Dynamic Arrival Rate Estimation for Campus Mobility on Demand Network Graphs

Mobility On Demand (MOD) systems are revolutionizing transportation in urban settings by improving vehicle utilization and reducing parking congestion. A key factor in the success of an MOD system is the ability to measure and respond to real-time customer arrival data. Real time traffic arrival rate data is traditionally difficult to obtain due to the need to install fixed sensors throughout the MOD network. This paper presents a framework for measuring pedestrian traffic arrival rates using sensors onboard the vehicles that make up the MOD fleet. A novel distributed fusion algorithm is presented which combines onboard LIDAR and camera sensor measurements to detect trajectories of pedestrians with a 90% detection hit rate with 1.5 false positives per minute. A novel moving observer method is introduced to estimate pedestrian arrival rates from pedestrian trajectories collected from mobile sensors. The moving observer method is evaluated in both simulation and hardware and is shown to achieve arrival rate estimates comparable to those that would be obtained with multiple stationary sensors.Comment: Appears in 2016 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). http://ieeexplore.ieee.org/abstract/document/7759357